It is known, for example from U.S. Pat. No. 5,389,903, to couple the input means of a bandpass filter of the aforementioned type to a first co-axial resonator element via a loop of wire. One end of the loop is connected to the filters input terminal within the filter housing; the other end of the loop is connected to an internal surface of the housing. The loop inductively couples the filter's input terminal to the first resonator element.
Such a known coupling arrangement provides good coupling properties. However it is difficult to finely adjust the impedance matching particularly for filters requiring tuning over a wide range of frequencies.
In such cases adjustment may be obtained by bending the loop with a probe inserted through an aperture in the filter housing to displace the loop in relation to the adjacent resonator element, or deforms it. Using this method, fine adjustment is difficult, particularly if the area in which the loop is located is too small to allow a safe bending action, as there is a likelihood of mechanical damage during the adjustment process.